The present invention relates to a collimator optical system by which an elliptical beam from a light source such as a semi-conductor laser that produces a beam having different beam divergence angles in a direction perpendicular to the active layer (i.e., vertical transverse mode) and in a direction parallel to said layer (i.e., horizontal transverse mode) can be converted to a circular beam of parallel rays.
A semiconductor laser commonly used in lightwave communications is a point source of light that produces an elliptical beam having different divergence angles in a direction perpendicular to the active layer (i.e., vertical transverse mode) and in a direction parallel to said layer (i.e., horizontal transverse mode). When a lightwave signal from such semiconductor laser is collimated with a conventional lens or a rod-shaped lens having a gradient of refractive index distribution from the center outward in their radial direction, the resulting beam is composed of parallel rays having an elliptical cross section. If such a beam is coupled to an optical device such as an optical fiber, it will not be efficiently introduced into the optical fiber having a circular cross section.
A beam that is divergent in an elliptical form may be collimated into parallel rays having a circular cross section by means of an optical system composed of two semi-cylindrical lenses crossed at right angles. However, because of the relation between curvature and thickness of each semi-cylindrical lens, this optical system is inevitably complex and it is very difficult to manufacture the desired lenses. In addition, except in special cases, it is impossible to construct a feasible system using semi-cylindrical lenses that are manufactured by the same process and which have the same dimensions.